Ore dake Haireru Kakushi Dungeon: Kossori Kitaete Sekai Saikyou

Synopsis

The third son of a “beggar noble” Nore lost his job and was at a loss. But as he was contemplating whether to become an adventurer, fortune struck. A Hidden Dungeon that no one knows how to enter. A legendary place full of hidden things like extremely rare magical beasts, magic tools for gaining power, and items that would be scarce all over the world. That’s the kind of entrance that was opened. “Without telling anyone about this place I’ll secretly become the strongest.” This world is harsh to nobles of poverty. Without the authority of honor, if I could at least acquire power――

Also known as 俺だけ入れる隠しダンジョン ～こっそり鍛えて世界最強～, Special training in the Secret Dungeon